These studies were designed to develop an animal model and a standardized assay for comparing and evaluating mycoplasma strain virulence and the potency of mycoplasmal vaccines. Syrian hamsters were selected because M. pneumoniae-induced pneumonia in these animals is similar to infection in patients. Colonization of infected hamster lungs was determined by culture procedures and the severity of lung disease was assessed by scoring the histopathologic lesions. Intratracheal inoculation was superior to the intranasal or aerosol routes for inducing a consistent degree of lung disease. A challenge dose of 10 to the 6th power CFU of M. pneumoniae strain Pl-1428, (p. 3), inoculated intratracheally, produced reproducible lung colonization and lung pathology scores. The peak infection occurred at about 2 weeks post-challenge. M. pneumoniae strains Pl-1428 and M129, at low passage (p. 3-6), were the most pathogenic whereas the high passage (p. 176), noncytadsorbing strain B176 did not colonize lungs nor cause lung lesions. Strain FH (p.50) still retained the ability to colonize respiratory tissues and produce a lesser amount of lung pathology. Animals younger than six weeks of age exhibited a lower lung pathology score than older animals. Thus, the hamster intratracheal infection model provides a reproducible amount of disease and the procedure should prove useful for the evaluation of M. pneumoniae vaccines. When M. genitalium, a human urogenital pathogen, that has cross-reacting antigenicity with M. pneumoniae, was inoculated intratracheally, the hamster lungs and trachea became colonized, but the histopathological changes were not typical of a M. pneumoniae pneumonia.